Coal-tar is a by-product produced during the destructive distillation of coal, known as carbonization or coking. The composition and properties of a coal-tar depend mainly on the temperature of the carbonization and, to a lesser extent, on the nature of the coal used as the feedstock. Coal-tars are usually black viscous liquids or semi-solids, with a characteristic, naphthalene-like odor. Compositionally, coal-tars are complex combinations of hydrocarbons, phenols and heterocyclic oxygen, sulphur and nitrogen compounds. Over 400 compounds have been identified in coal-tars, and it has been estimated that probably as many as 10,000 are actually present.
As is known to those skilled in the art, the relative proportions of the various coal-tar components is quite different in tars made by low-temperature processes compared with those made by high-temperature processes, the former having a higher content of phenols and tar acids and a lower content of medium-soft pitch. High-temperature coal-tars are condensation products obtained by the cooling of gases which evolve in the high-temperature (&gt;700.degree. C.) carbonization of coal. High temperature coal-tars are of two main types: coke-oven tars and continuous vertical-retort (CVR) tars. Coke ovens for blast furnaces use the highest temperature (1250-1350.degree. C.)., while a slightly lower temperature (1000-1100.degree. C.) is applied in continuous vertical-retorts for the manufacture of domestic heating coke and gas.
The aromaticity of coal-tars increases and the content of paraffins and phenols decreases when the carbonization temperature increases. Thus, coke-oven tars contain relatively small amounts of aliphatic hydrocarbons, whereas CVR tars contain a higher proportion of straight-chain or slightly branched-chain paraffins (about 20% in the lower-boiling fractions of tar to 5-10% in the higher distillate oils). Coke-oven tars contain about 3% of phenolic compounds in the fractions distilling at up to 300.degree. C. CVR tars, on the other hand, contain 20-30% of phenolic compounds. Both aromatic and heterocyclic rings occur in substituted and unsubstituted forms. The aromatic compounds in CVR tars are mostly alkyl derivatives, whereas coke-oven tars consist predominately of compounds containing unsubstituted rings.
The polynuclear aromatic hydrocarbon (PNA) profile of coal-tars is relatively independent of the starting material, being mainly a function of temperature. PNA concentrations found in high-temperature coal-tars may typically exceed 25%, with levels of benzo[a]pyrene often exceeding 0.5%. In coking experiments, it has been observed that the formation of benzo[a]pyrene starts at 700.degree. C. and increases with temperature. At 1100.degree. C., the benzo[a]pyrene content of the coal-tar is usually about 0.2%.
Low temperature coal-tars are the condensation products obtained by the cooling of gases which evolve in the low-temperature (&lt;700.degree. C.) carbonization of coal. They are black viscous liquids, more dense than water, and are less aromatic than high-temperature coal-tars. The content of aromatic hydrocarbons, usually alkyl-substituted, is only 40-50%. Low-temperature coal-tars also contain 30-35% of non-aromatic hydrocarbons and about 30% of alkali-extractable phenolic compounds in their distillate oils.
A wide variety of coal-tar-derived products exist, each serving a particular class of specialty applications. The distillate fractions, also known as tar oils, or creosote, are obtained by the fractional distillation of crude coal-tars. Creosotes are primarily used for timber preservation. Creosote is also used as an animal or bird repellent, animal dip, miticide, fungicide, herbicide and insecticide. Another product, anthracene oil is a semi-solid, greenish-brown crystalline material. Anthracene oil is obtained from the primary distillation of coal-tars in two fractions. The lower-boiling fraction (light anthracene oil) has a high content of phenanthrene, anthracene and carbazole; the higher-boiling fraction (heavy anthracene oil) has a high content of fluoranthene and pyrene. Benzo[a]pyrene concentrations range between 0.01 and 0.06% in anthracene oil.
Coal-tar pitch is a dark-brown-black, shiny, amorphous residue produced during the distillation of coal-tars. Pitch is composed of many different compounds which interact to form eutectic mixtures; consequently it does not show a distinct melting or crystallization point. Rather, it is characterized by its softening point. Depending on the depth of distillation, pitches with different softening points can be obtained. Pitch contains PNA's and their methyl and polymethyl derivatives, as well as heteronuclear compounds. When coal is carbonized to make coke and/or gas, crude coal-tar is one of the by-products. The low temperature processes are used to produce solid smokeless fuels for industrial and home heating.
As may readily be appreciated, the production of coal-tars is closely linked with steel production, because of the need for coke in steel making. Coal-tar is suitable for burning as a fuel in the steel industry in open-hearth furnaces and blast furnaces because of its availability, its low sulphur content and its high heating value. Both high-temperature and low-temperature coal-tars are used topically in the treatment of psoriasis and other chronic skin diseases. Coal-tar products are available in many pharmaceutical vehicles, including creams, ointments, pastes, lotions, bath and body oils, shampoos, soaps and gels. Shampoos are the most important of the products. The USP grade of coal-tar is used in denatured alcohol. Also, several surface-coating formulations contain coal-tar at varying concentrations.
The major use for coal-tar pitches is as a binder for aluminum smelting electrodes. Pitches are also used in roofing, surface coatings, for pitch coke production and for a variety of other applications. While bitumen is a more usual roofing material, coal-tar pitch is also used for this purpose. When used for roofing, the coal-tar pitch is heated and applied at approximately 200.degree. C. Coal-tar pitch is also used in surface coatings. Black varnishes, which are soft pitches fluxed usually with heavy anthracene oil, are sometimes used as protective coatings for industrial steelwork and as an antifouling paint for marine applications. Pipe-coating enamels, made by fluxing a coke-oven pitch with anthracene oil, are used to protect buried oil, gas and water pipes from corrosion. About 75% of all underground petroleum, gas and municipal water pipelines are coated with coal-tar enamels. Coal-tar pitch is also used to impregnate and strengthen the walls of brick refractories, while target pitch, a very hard pitch, is used with a clay or limestone filler to produce brittle clay pigeons used for target practice.
Low-temperature pitch is also used as a road binder. Tar/bitumen blends, which may be polymer-modified, are used as surface-dressing binders. Coal-tar pitch is also used in the production of smokeless, precarbonized briquettes containing 8-10% of medium-soft coke-oven pitch.
Refined tar, which is made by fluxing a high-temperature pitch to a low softening-point with strained anthracene or heavy oils, is used as an extender for resins, including epoxy and polyurethane. Such formulations are used to produce abrasion-resistant, waterproof films, which are used for coating storage tanks, marine pilings and bridge decks. They are highly resistant to petroleum-based fuels. Soft coal-tar pitch is used to impregnate paper tubes to produce pitch-fiber pipes for the transport of sewage and effluents and for irrigation purposes.
As is readily evident, a wide variety of important applications exist for coal-tar-derived products. However, in recent years, concerns have arisen regarding the potential hazards associated with the use of various coal-tar-derived products containing polynuclear aromatics (PNA's), since certain of these compounds have been shown to cause cancer in humans and laboratory animals following exposure thereto, particularly those materials having high PNA levels.
To determine the relative mutagenic activity of a coal-tar-based product, a reliable test method for assaying such activity in complex hydrocarbon mixtures is required. A highly reproducible method showing strong correlation with the carcinogenic activity index of hydrocarbon mixtures is disclosed in U.S. Pat. No. 4,499,187, which is incorporated by reference in its entirety. From the testing of hydrocarbon samples as disclosed in U.S. Pat. No. 4,499,187, a property of the sample, known as its Mutagenicity Index (MI) is determined. Samples exhibiting MI's less than or equal to 1.0 are known to be non-carcinogenic, while samples exhibiting MI's equal to about 0.0 are known to be completely free of mutagenic activity.
U.S. Pat. No. 5,034,119 discloses a process for producing non-carcinogenic bright stock extracts and deasphalted oils from reduced hydrocarbon feedstocks. Such non-carcinogenic products are produced by establishing a functional relationship between mutagenicity index and a physical property correlative of hydrocarbon type for the bright stock extract or deasphalted oil and determining a critical physical property level which, when achieved, results in a product having a mutagenicity index of less than about 1.0. Process conditions are established so that a product stream achieving the desired physical property level can be produced. Non-carcinogenic bright stock extracts or deasphalted oils are then processed utilizing the conditions so established.
Despite these advances in the art, a need exists for a process for reducing the mutagenicity of coal-tar-based products.